This invention relates to the synthesis of ammonia from molecular nitrogen and hydrogen through the use of a new catalyst and novel processes based thereon. Ammonia is an important raw material in the chemical industry, particularly in the production of synthetic fertilizers. Agricultural research has shown that nitrogen is an indispensable ingredient of fertilizers for crops. The major source of that nitrogen at the present time is ammonia and one of the major goals of chemical technology in the fertilizer field is the production of ammonia at faster rates and with correspondingly lower costs.
Prior art methods for the production of ammonia from gaseous nitrogen and hydrogen have employed iron catalysts of various types. However, such catalysts require high temperatures and pressures necessitating expensive equipment of relatively low capacity, and are rapidly consumed at such temperatures and pressures by various degradation mechanisms.
Research on the synthesis of ammonia has also progressed in the direction of room temperature synthesis in aqueous solutions where either biological "nitrogen-fixation" conditions are simulated, or a metal salt or complex is used as a catalyst together with a reducing agent. Although aqueous studies are still at the fundamental research level and have not yet been commercialized, it has been found that the metals of heterogeneous catalysts effective to synthesize ammonia at high temperatures from gaseous nitrogen and hydrogen also catalyze the ammonia synthesis when present as a salt or complex in aqueous solutions.
Although gaseous nitrogen and hydrogen theoretically can be combined at room temperature and atmospheric pressure to produce ammonia at an equilibrium yield of 95.5% on thermodynamic grounds, there is no catalyst or method for ammonia synthesis in the prior art that can be employed at room temperature to produce ammonia from gaseous nitrogen and hydrogen at commercially feasible rates. Therefore, it has been necessary to employ much higher temperatures in the synthesis of ammonia to achieve satisfactory reaction rates. The iron catalysts previously used produce no appreciable ammonia from gaseous nitrogen and hydrogen at temperatures below approximately 360.degree. C. Even higher temperatures are therefore necessary for acceptable yields. However, higher temperatures in turn result in a drastic reduction in the thermodynamic equilibrium yield of the reaction due to its exothermic nature. Reduction in equilibrium yields with increasing temperatures can only be partly compensated for by increasing the operating pressure, and the pressures needed closely approach the design limits of the equipment presently available for industrial application.
Prior art processes for the synthesis of ammonia from gaseous nitrogen and hydrogen over commercial iron catalysts operate at temperatures around 500.degree. C. The equilibrium yield of the reaction at this temperature with only one atmosphere of pressure is well below 0.1%. Higher pressures in the range of 150 atmospheres are therefore employed to compensate for the low yield at ambient pressure. Such high pressures result in high equipment and maintenance cost. Furthermore, equilibrium yields attained at such elevated pressures are usually less than 15%. This means that a substantial portion of the effluents from reactor vessels must be recycled one or more times, adding substantially to the cost of production both in the form of added equipment and longer operating times for separation and recycle of the product stream.
By reason of the foregoing thermodynamic and kinetic considerations, the cost of producing ammonia by prior art methods is quite high, involving relatively slow production rates and costly equipment. The cost of the iron catalyst itself is also quite substantial, mainly because special manufacturing processes must be utilized to improve the catalytic properties of the iron. Thus, additional components called promoters must be added to the iron and, in most cases, both the iron and the promoters must be supported on a special carrier. The addition of the catalytic constituents to the carrier is a manner to permit sufficient contact between those constituents and the gaseous reactants is quite expensive.